Typically, a vehicle electric control unit (ECU) has various types of vehicle information such as fault information, a learned value for vehicle control, and a vehicle identification number (hereinafter, referred to as “VIN code”) that is uniquely assigned to a vehicle to identify the vehicle. For example, if the VIN code is lost, the vehicle cannot be identified. Also, if the learned value is lost, vehicle control function may be degraded, and running performance and emission performance of the vehicle may be reduced accordingly. Therefore, a nonvolatile memory such as an electrically erasable programmable read only memory (EEPROM) is used to store such vehicle information so that the vehicle information can be retained even when the ECU is disconnected from a vehicle battery. It is noted that on-board diagnostics (OBD) regulations specify that the fault information should be stored in a nonvolatile memory.
Further, the OBD regulations specify that fault information related to emission of the vehicle should be cleared (i.e., erased) from the memory at the same time when the VIN code is overwritten.
The fact that the VIN code stored in the memory of the ECU is overwritten generally means that the ECU is transferred to another vehicle. Therefore, although the OBD regulations specify nothing about vehicle individual information such as the learning value, it is preferable that the vehicle individual information be cleared (i.e., reset to an initial value) at a time when the VIN code is overwritten. For example, U.S. Pat. No. 6,195,602 corresponding to JP-A-H11-255079 discloses that a VIN code is stored in a nonvolatile memory and that a control program is overwritten at a time when the VIN code is overwritten.
In some cases, the VIN code may be overwritten despite the fact that the ECU remains on the same vehicle. For example, when the VIN code is broken and thus becomes abnormal, the VIN code needs to be corrected by overwriting the abnormal VIN code with a normal VIN code. In this case, the ECU is not transferred to another vehicle. Therefore, it is preferable that the vehicle individual information such as the learned value be not cleared, because the vehicle individual information can continue to be used by the ECU. If the vehicle individual information is cleared despite the fact that the ECU remains on the same vehicle, the vehicle running performance and emission performance may be reduced unnecessarily.